Watching and Reporting On Global News and Events

Messenger Data from Mercury Orbit Confirms Theories, Offers Surprises

NASA scientists are making new discoveries about the planet Mercury. Data from MESSENGER, the first spacecraft to orbit Mercury, is giving scientists important clues to the origin of the planet and its geological history and helping them better understand its dynamic interior and exterior processes.

NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft, or MESSENGER, has been orbiting Mercury since March 18. To date the spacecraft has provided tens of thousands of images showing detailed planetary features. The planet’s surface previously had been seen only at comparatively low resolution but is now in sharper focus.

The spacecraft also has collected extensive measurements of the chemical composition of Mercury’s surface and topography and gathered global observations of the planet’s magnetic field. Data now confirm that bursts of energetic particles in Mercury’s magnetosphere are a continuing product of the interaction of Mercury’s magnetic field with the solar wind.

“We are assembling a global overview of the nature and workings of Mercury for the first time,” said MESSENGER principal investigator Sean Solomon of the Carnegie Institution of Washington. “Many of our earlier ideas are being cast aside as new observations lead to new insights. Our primary mission has another three Mercury years to run, and we can expect more surprises as our solar system’s innermost planet reveals its long-held secrets.”

Flyby images of Mercury had detected bright, patchy deposits on some crater floors. Without high-resolution images to obtain a closer look, these features remained only a curiosity. Now new detailed images have revealed these patchy deposits to be clusters of rimless, irregular pits varying in size from several hundred feet to a few miles wide. These pits are often surrounded by diffuse halos of more reflective material and are found on central peaks, peak rings, and rims of craters.

“The etched appearance of these landforms is unlike anything we’ve seen before on Mercury or the moon,” said Brett Denevi, a staff scientist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., and a member of the MESSENGER imaging team. “We are still debating their origin, but they appear to be relatively young and may suggest a more abundant than expected volatile component in Mercury’s crust.”

One of two instruments on the spacecraft designed to measure the quantity of key chemical elements on Mercury has made several important discoveries since the orbital mission began. Elemental ratios averaged over large areas of the planet’s surface show that Mercury’s surface differs markedly in composition from that of the moon.

Observations have revealed substantial amounts of sulfur at Mercury’s surface, lending support to prior suggestions from ground-based telescopic observations that sulfide minerals are present. This discovery suggests that the original building blocks from which Mercury formed may have been less oxidized than those that formed the other terrestrial planets. The result also hints that sulfur-containing gases may have contributed to past explosive volcanic activity on Mercury.

Topography data of Mercury’s northern hemisphere reveal the planet’s large-scale shape and profiles of geological features in high detail. The north polar region is a broad area of low elevations, whereas the overall range in topographic heights seen to date exceeds 5 miles (9 kilometers).

Two decades ago, Earth-based radar images showed deposits thought to consist of water ice and perhaps other ices near Mercury’s north and south poles. These deposits are preserved on the cold, permanently shadowed floors of high-latitude impact craters. MESSENGER is testing this idea by measuring the floor depths of craters near Mercury’s north pole. The craters hosting polar deposits appear to be deep enough to be consistent with the idea that those deposits are in permanently shadowed areas.

During the first of three Mercury flybys in1974, Mariner 10 discovered bursts of energetic particles in the planet’s Earth-like magnetosphere. Four bursts of particles were observed on that flyby. Scientists were puzzled that no such strong events were detected by MESSENGER during any of its three flybys of the planet in 2008 and 2009. But now that the spacecraft is in near-polar orbit around Mercury, energetic events are being seen regularly.

A Surface Revealed in Unprecedented Detail

As part of MESSENGER’s global imaging campaign, the Mercury Dual Imaging System (MDIS) is acquiring global monochrome and stereo base maps with an average resolution of 250 meters per pixel and a global color base map at an average of 1.2 kilometer per pixel. These base maps are providing the first global look at the planet under optimal viewing conditions.

Orbital images reveal broad expanses of smooth plains near Mercury’s north pole. Flyby images from MESSENGER and from Mariner 10 in the 1970s indicated that smooth plains may be important near the north pole, but much of the territory was viewed at unfavorable imaging conditions.

MESSENGER’s new orbital images show that the plains are likely among the largest expanses of volcanic deposits on Mercury, with thicknesses of up to several kilometers. The broad expanses of plains confirm that volcanism shaped much of Mercury’s crust and continued through much of Mercury’s history, despite an overall contractional stress state that tended to inhibit the extrusion of volcanic material onto the surface.

Among the fascinating features seen in flyby images of Mercury were bright, patchy deposits on some crater floors. Without high-resolution images to obtain a closer look, these features remained only a curiosity. New targeted MDIS observations at up to 10 meters per pixel reveal these patchy deposits to be clusters of rimless, irregular pits varying in size from hundreds of meters to several kilometers. These pits are often surrounded by diffuse halos of higher-reflectance material, and they are found associated with central peaks, peak rings, and rims of craters.

“The etched appearance of these landforms is unlike anything we’ve seen before on Mercury or the Moon,” says Brett Denevi, a staff scientist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., and a member of the MESSENGER imaging team. “We are still debating their origin, but they appear to have a relatively young age and may suggest a more abundant than expected volatile component in Mercury’s crust.”

Mercury’s Surface Composition

The X-Ray Spectrometer (XRS) — one of two instruments on MESSENGER designed to measure the abundances of many key elements on Mercury — has made several important discoveries since the orbital mission began. The magnesium/silicon, aluminum/silicon, and calcium/silicon ratios averaged over large areas of the planet’s surface show that, unlike the surface of the Moon, Mercury’s surface is not dominated by feldspar-rich rocks.

XRS observations have also revealed substantial amounts of sulfur at Mercury’s surface, lending support to prior suggestions from ground-based telescopic spectral observations that sulfide minerals are present. This discovery suggests that the original building blocks from which Mercury was assembled may have been less oxidized than those that formed the other terrestrial planets, and it has potentially important implications for understanding the nature of volcanism on Mercury.

MESSENGER’s Gamma-Ray and Neutron Spectrometer has detected the decay of radioactive isotopes of potassium and thorium and has allowed a determination of the bulk abundances of these elements. “The abundance of potassium rules out some prior theories for Mercury’s composition and origin,” says Larry Nittler, a staff scientist at the Carnegie Institution of Washington. “Moreover, the inferred ratio of potassium to thorium is similar to that of other terrestrial planets, suggesting that Mercury is not highly depleted in volatiles, contrary to some prior ideas about its origin.”

Mapping of Mercury’s Topography and Magnetic Field

MESSENGER’s Mercury Laser Altimeter has been systematically mapping the topography of Mercury’s northern hemisphere. After more than two million laser-ranging observations, the planet’s large-scale shape and profiles of geological features are both being revealed in high detail. The north polar region of Mercury, for instance, is a broad area of low elevations.

The overall range in topographic heights seen to date exceeds 9 kilometers.
Two decades ago, Earth-based radar images showed that near both Mercury’s north and south poles are deposits characterized by high radar backscatter. These polar deposits are thought to consist of water ice and perhaps other ices preserved on the cold, permanently shadowed floors of high-latitude impact craters. MESSENGER’s altimeter is testing this idea by measuring the floor depths of craters near Mercury’s north pole. To date, the depths of craters hosting polar deposits are consistent with the idea that those deposits occupy areas in permanent shadow.

The geometry of Mercury’s internal magnetic field can potentially discriminate among theories for how the field is generated. An important finding is that Mercury’s magnetic equator, determined on successive orbits as the point where the direction of the internal magnetic field is parallel to the spin axis of the planet, is well north of the planet’s geographic equator. The best-fitting internal dipole magnetic field is located about 0.2 Mercury radii, or 480 km, northward of the planet’s center. The dynamo mechanism in Mercury’s molten, metallic outer core responsible for generating the planet’s magnetic field therefore has a strong north-south asymmetry.

As a result of this north-south asymmetry, the geometry of magnetic field lines is different in Mercury’s north and south polar regions. In particular, the magnetic “polar cap” where field lines are open to the interplanetary medium is much larger near the south pole. This geometry implies that the south polar region is much more exposed than in the north to charged particles heated and accelerated by solar wind–magnetosphere interactions. The impact of those charged particles onto Mercury’s surface contributes both to the generation of the planet’s tenuous atmosphere and to the “space weathering” of surface materials, both of which should have a north-south asymmetry given the different magnetic field configurations at the two poles.

Energetic Particle Events at Mercury

One of the major discoveries made by Mariner 10 during the first of its three flybys of Mercury in 1974 were bursts of energetic particles in Mercury’s Earth-like magnetosphere. Four bursts of particles were observed on that flyby, so it was puzzling that no such strong events were detected by MESSENGER during any of its three flybys of the planet in 2008 and 2009.

With MESSENGER now in near-polar orbit about Mercury, energetic events are being seen almost like clockwork, says MESSENGER Project Scientist Ralph McNutt, of APL. “While varying in strength and distribution, bursts of energetic electrons — with energies from 10 kiloelectron volts (keV) to more than 200 keV — have been seen in most orbits since orbit insertion,” McNutt says. “The Energetic Particle Spectrometer has shown these events to be electrons rather than energetic ions, and to occur at moderate latitudes. The latitudinal location is entirely consistent with the events seen by Mariner 10.”

With Mercury’s smaller magnetosphere and with the lack of a substantial atmosphere, both the generation of these energetic electrons and their distribution are different than at Earth. One candidate mechanism for the generation of these energetic electrons is the formation of a “double layer,” a plasma structure with large electric fields along the local magnetic field. Another is induction brought about by rapid changes in the magnetic field, a process that follows the principle used in generators on Earth to produce electric power. Which of these mechanisms, if either, predominates in the acceleration of energetic electrons will be the subject of study over the coming months.

“One mystery has been answered, only to be replaced by another, but that is how science works,” McNutt says. “In the coming months as MESSENGER’s orbit swings around the planet, we will be able to observe the overall geometry of these events, providing yet more clues to their production and interaction with the planet.”

“We are assembling a global overview of the nature and workings of Mercury for the first time,” adds Solomon, “and many of our earlier ideas are being cast aside as new observations lead to new insights. Our primary mission has another three more Mercury years to run, and we can expect further surprises as our solar system’s innermost planet reveals its long-held secrets.”

MESSENGER is currently filling in coverage of Mercury’s north polar region, which was seen only partially during the Mariner 10 and MESSENGER flybys. Flyby images indicated that smooth plains were likely important in Mercury’s northernmost regions. MESSENGER’s orbital images show that the plains are among the largest expanses of volcanic deposits on Mercury, with thicknesses of several kilometers in many places. The estimated extent of these plains is outlined in yellow. This mosaic is a combination of flyby and orbital coverage in a polar stereographic projection showing latitudes from 50° to 90° N. The longitude at the 6 o’clock position is 0°.

Although image coverage of Mercury from the MESSENGER and Mariner 10 flybys extended to approximately 98% of the surface, such images were obtained at varying resolution and lighting and often at extreme viewing geometries. This scene at 74° N, 336° E, shows a region viewed during MESSENGER’s second flyby, but its high-latitude location made the recognition of surface features a difficult task. The scene is approximately 275 km from top to bottom.
Image 1.2b Advantages of a global perspective: the same example

MESSENGER’s orbital images have been overlaid on an image from the second flyby shown in Image 1.2a. Even for previously imaged portions of the surface, orbital observations reveal a new level of detail. This region is part of the extensive northern plains, and evidence for a volcanic origin can now be seen. Several examples of “ghost” craters, preexisting craters that were buried by the emplacement of the plains, are seen near the center of the mosaic.

An example of MESSENGER’s color base map imaging campaign, which will collect global images acquired through eight filters between wavelengths of 430 and 1000 nm at an average resolution of 1.2 km/pixel. The northern plains are seen here to be distinctive in color and thus composition from the surrounding terrain. The mosaicked images are shown with the 1000, 750, and 430 nm images in red, green, and blue, respectively. The scene is centered at 73° N, 300° E.

The same scene as that in Image 1.3a is shown after the application of a statistical method that highlights differences among the eight color filters, making variations in color and composition easier to discern.

Imaging for MESSENGER’s global monochrome base map, collected at an average resolution of 250 m/pixel, is well underway. This animation shows the current monochrome coverage, centered at 240° E, and zooms in to highlight one of the many striking features seen during the first few months of orbital operations. At 9.0° S, 254.7° E, impact melt flowed from an unnamed, 13-km-diameter impact crater, extending outward more than one crater diameter from the rim. In contrast to impact melt observed at other craters on Mercury, this flow is low in reflectance and appears to be compositionally distinct from its surroundings—a confirmation that Mercury’s crust is heterogeneous both vertically and laterally on relatively small scales.

This spectacular view of the crater Degas was obtained as a high-resolution targeted observation (90 m/pixel). Impact melt coats its floor, and as the melt cooled and shrank, it formed the cracks observed across the crater. For context, Mariner 10’s view of Degas is shown at left. Degas is 52 km in diameter and is centered at 37.1° N, 232.8° E.

Previous images hinted at unusual high-reflectance features associated with impact crater floors. High-resolution (21 m/pixel) monochrome images reveal these features to be rimless, irregular pits varying in size from hundreds of meters to up to several kilometers. These pits are often surrounded by diffuse halos of higher-reflectance material, and they are found associated with central peaks, peak rings, and rims of craters. The unusual etched appearance of these landforms may suggest a higher than expected volatile component in Mercury’s crust, and their sharp features are consistent with a relatively young age. The mosaic shown here is centered at 44.0° N, 290.9° E.

Schematic illustration of the operation of MESSENGER’s X-ray Spectrometer (XRS). When X-rays emitted from the Sun’s corona strike the planet, they can induce X-ray fluorescence from atoms at the surface. Detection of these fluorescent X-rays by the XRS allows determination of the surface chemical composition.

Major-element composition of Mercury’s surface materials, depicted on the same graph, as measured by the MESSENGER XRS. Mercury has lower Al/Si and higher Mg/Si than typical lunar surface materials and terrestrial basalts, indicating a lower fraction of the common mineral plagioclase feldspar.

Schematic illustration of the operation of MESSENGER’s Gamma-ray and Neutron Spectrometer (GRNS). Galactic cosmic rays interact with the surface of Mercury to a depth of tens of centimeters, producing high-energy (“fast”) neutrons. These neutrons further interact with surface material, resulting in the emission of gamma-rays with energies characteristic of the emitting elements and low-energy (“slow”) neutrons. Naturally occurring radioactive elements such as potassium (K), thorium (Th), and uranium (U) also emit gamma-rays. Detection of the gamma-rays and neutrons by GRNS allows determination of the chemical composition of the surface.

Ratio by weight of potassium to thorium for the terrestrial planets and Moon, plotted versus distance from the Sun. Because K is a volatile element and Th a refractory one, this ratio is a sensitive measure of thermal processes that fractionate elements by volatility. For example, the ratio for the Moon (360) is much lower than that for Earth (3000), reflecting volatile loss during the Moon’s formation by a giant impact. The ratio for Mercury (~6000), determined by GRNS, is comparable to that of Venus, Earth, and Mars, indicating that Mercury is not highly depleted in volatile elements, ruling out some models for its formation and early history.

Schematic illustration of the operation of MESSENGER’s Mercury Laser Altimeter (MLA). MLA ranges to Mercury whenever the spacecraft is within 1,800 km of the surface. Eight times per second, MLA’s laser emits a short (5 ns) pulse, which propagates along the laser transmitter’s line of sight to the surface, where a fraction of the pulse energy is reflected from the surface and propagates back to MLA’s four receiver telescopes. The time of flight of the laser pulse provides the distance to the portion of the surface from which signal was reflected. Knowledge of the spacecraft’s position then allows recovery of the elevation of the reflection point.

This animation shows the MLA profiles acquired between 29 March and 24 May. Elevation along track (relative to a sphere of radius 2,440 km) is shown by the color scale. Note the broad area of generally low elevations in Mercury’s north polar region. The total range in elevation measured by MLA to date is more than 9 km. Orthographic projection centered on the north pole; the outmost circle corresponds to Mercury’s equator.

A portion of a radar image of Mercury’s north polar region (latitudes indicated) obtained at the Arecibo Observatory. The bright features are polar deposits, areas of high radar backscatter thought to consist of water ice and perhaps other ices preserved on the cold, permanently shadowed floors of high-latitude impact craters.

A portion of a northern hemisphere mosaic of Mercury’s surface (500 m/pixel) on which the radar image in Image 3.3 has been superposed. The prominent impact crater circled in red hosts an area of polar deposits and was profiled several times by MLA early in MESSENGER’s science mapping phase. The crater is centered at 82.3°N, 342.8°E, and is 24 km in diameter.

This topographic contour map was constructed from the several MLA profiles (lines of white circles) that pass through and near the crater circled in Image 3.4. The color scale at right is in km, and north is at the 4 o’clock position. Calculations show that the topography of the crater is consistent with the prediction that the southernmost portion of the crater floor is in permanent shadow.

This animation shows the location of Mercury’s magnetic equator determined on successive orbits as the point where the direction of the internal magnetic field is parallel to the spin axis of the planet. This magnetic equator is well north of the planet’s geographic equator (indicated by the horizontal gray line). The best-fitting internal dipole magnetic field is located about 0.2 Mercury radii, or 480 km, northward of the planet’s center. The dynamo mechanism in Mercury’s molten outer core responsible for generating the planet’s magnetic field therefore has a strong north-south asymmetry.

Click on image to play the movie. (Macs, view with FireFox)

Image 3.7a Magnetic field lines differ at Mercury’s north and south poles

As a result of the north-south asymmetry in Mercury’s internal magnetic field, the geometry of magnetic field lines is different in Mercury’s north and south polar regions. In particular, the magnetic “polar cap” where field lines are open to the interplanetary medium is much larger near the south pole. This geometry implies that the south polar region is much more exposed than in the north to charged particles heated and accelerated by solar wind–magnetosphere interactions. The impact of those charged particles onto Mercury’s surface contributes both to the generation of the planet’s tenuous atmosphere and to the “space weathering” of surface materials, both of which should have a north-south asymmetry given the different magnetic field configurations at the two poles.

MESSENGER’s Gamma-Ray Spectrometer (GRS), although designed to identify elements in Mercury’s surface material, can also respond to energetic electrons. In this instrument schematic, the “entrance” at the top of the drawing normally points toward Mercury’s surface. The enlargement depicts a schematic of the active sensor, a polished single crystal of the semiconducting element germanium, held at a constant temperature of 90 K (-297° F) by the instrument’s cooler. The crystal is 5 cm (2 inches) in both height and diameter.

The GRS casing is sufficiently thick to stop all of the electrons, but the impacts yield X-rays in much the same way that medical X-ray machines operate. A fraction of those X-rays reach the germanium detector, where they are registered and the data subsequently transmitted to Earth as part of the normal gamma-ray data. From these data, the properties of the electrons at the spacecraft can be deduced. The GRS channels affected are at energies well below those at which gamma rays from the planet are registered. The large size of the germanium crystal allows for higher time and energy resolution of the events than is possible with MESSENGER’s Energetic Particle Spectrometer (EPS). EPS observations provide information on the direction of the events.

Click on image to enlarge.

Image 4.3 Distribution of energetic electron events recorded by MESSENGER

This animation shows the locations at which energetic electrons were detected by MESSENGER between March 24 and June 3, 2011. In the coordinate system used (known as the Mercury solar orbital, or MSO, coordinates), the Sun is always in a fixed direction (here +X-axis). The events are well distributed in local time, but most are seen when the spacecraft is in the northern hemisphere. MESSENGER’s X-ray Spectrometer (XRS) responds to electrons initially with lower energies but in a manner analogous to the GRS.

A cross-section of Mercury’s magnetosphere (in the noon-midnight plane, i.e., the plane containing the planet-Sun line and Mercury’s spin axis) provides context for the energetic electron events observed to date with the MESSENGER XRS and GRS high-purity germanium (HpGe) detectors. The Sun is toward the right; dark yellow lines indicate representative magnetic field lines. Blue and green lines trace the regions along MESSENGER’s orbit from April 2 to April 10 during which energetic electrons were detected and MESSENGER’s orbit was within ± 5° of the noon-midnight plane. The presence of events on the dayside, their lack in the southern hemisphere, and their frequency of occurrence at middle northern latitudes over all longitudes point to a more complex picture of magnetospheric activity than found at Earth.

Click on image to enlarge.

The spacecraft was designed and built by APL. The lab manages and operates the mission for NASA’s Science Mission Directorate (SMD) in Washington. The mission is part of NASA’s Discovery Program, managed for SMD by the agency’s Marshall Space Flight Center in Huntsville, Ala.

Translation

VPSDime penetrates the lean high performance low cost VPS market by providing a specification unrivaled for a price previously thought impossible. High octane servers at an extremely low price point makes it easier than ever for anyone looking to have lots of room to breath or run high intensity applications.

TheWatchTowers.com is is intended for Mature Audiences. Familiarize yourself with our legal and use policies every time you engage the site: they're updated constantly without notice. Do not use this site unless you agree to our serving cookies unless they're disabled on your browser (instructions included). Furthermore the content on this site has many origins and is provided for entertainment and or educational purposes. We site this for legal reasons only. Thank you for visiting.